Pulp is a composition comprising lignocellulosic fibrous material prepared by chemically or mechanically separating cellulose fibers from biomass, such as wood, fiber crops or waste paper. The timber resources used to make wood pulp are referred to as pulpwood. Wood pulp comes from softwood trees such as spruce, pine, fir, larch and hemlock, and hardwoods such as eucalyptus, aspen and birch.
A pulp mill is a manufacturing facility that converts wood chips or other plant fiber source into a thick fiberboard which can be shipped to a paper mill for further processing. Alternatively, pulp and paper facilities may be integrated and wet pulp mass can be used directly for paper production.
Pulp is characterized by its ability to absorb and retain water, which may be quantified as Canadian Standard Freeness (CSF) measured in milliliters. Defibrated wood material can be considered as pulp if its CSF can be determined.
Pulp can be manufactured using mechanical, semi-chemical or fully chemical methods (Kraft and sulfite processes). The finished product may be either bleached or non-bleached, depending on the customer's requirements.
Wood and other plant materials that may be used to make pulp contain three main components (apart from water): cellulose fibers (desired for papermaking), lignin (a three-dimensional polymer that binds the cellulose fibers together) and hemicelluloses, (shorter branched carbohydrate polymers).
The aim of the pulping process is to break down the bulk structure of the fiber source, be it chips, stems or other plant parts, into the constituent fibers.
Chemical pulping such as Kraft pulping achieves this by chemically degrading the lignin and hemicellulose into small, water-soluble molecules which can be washed away from the cellulose fibers without depolymerizing the cellulose fibers. However, this process of chemically depolymerizing the hemicellulose weakens the fibers.
The Kraft process (also known as kraft pulping or sulfate process) is a process for conversion of wood into wood pulp, which consists of almost pure cellulose fibers. The Kraft process entails treatment of wood chips with a hot mixture of water, sodium hydroxide, and sodium sulfide, known as white liquor, which breaks the bonds that link lignin, hemicellulose, and cellulose. The technology entails several steps, both mechanical and chemical. It is the dominant method for producing paper.
The various mechanical pulping methods, such as groundwood (GW) and refiner mechanical pulping (RMP), physically tear the cellulose fibers one from another. Much of the lignin remains adhered to the fibers. Strength may also be impaired because the fibers may be cut.
There are a number of related hybrid pulping methods that use a combination of chemical and thermal treatment, for instance an abbreviated chemical pulping process, followed immediately by a mechanical treatment to separate the fibers. These hybrid methods include chemi-thermomechanical pulping, also known as CTMP. The chemical and thermal treatments reduce the amount of energy subsequently required by the mechanical treatment, and also reduce the loss of strength suffered by the fibers.
Mechanical pulping of wood is extremely energy intensive process; for example, a typical newsprint pulp may need 2160 kWh of refiner energy per ton of feedstock to refine wood chips into pulp. Reducing this energy requirement is a very acute need of the industry.
Lignin is the second most abundant biopolymer on the earth and a major component of the plant cell wall. Lignin is also a major waste product for several industries, including the paper and pulping industry and the lignocellulosic biorefinery. Due to the recalcitrant nature of the complex polyphenolic structure, the utilization of lignin for the production of biofuels and bioproducts is a major challenge for both biorefineries and paper/pulping industry. As compared to cellulose and hemicellulose, the methods and systems for utilization of lignin are very limited.
As one of the solutions, enzymes capable of oxidizing lignin were proposed to be used for pretreatment of wood chips in order to decrease the energy required for grinding. This idea was perceived from natural observation that fungi, especially white-rot fungi are able to decay wood material by secreting lignolytic enzymes such as peroxidases and laccases.
This idea was first implemented as so-called bio-pulping, when fungal species were actually cultivated on wood chips before pulping. This resulted in substantial energy saving, but cultivation time comprised several weeks, which was not acceptable in industrial context.
Subsequently, it was proposed to use isolated enzyme preparations for wood pretreatment, rather than live species, which should in principle produce similar effect. This resulted in a limited number of publications wherein isolated fungal enzymes, such as laccases and xylanases were employed for wood chips pretreatment.
Bleaching of wood pulp is the chemical processing carried out on various types of wood pulp to decrease the color of the pulp, so that it becomes whiter. This process requires degradation or removal of the residual lignin, which causes the color. The main use of wood pulp is to make paper where whiteness or “brightness” is an important characteristic. The processes and chemistry described herein are also applicable to the bleaching of non-wood pulps, such as those made from bamboo or kenaf.
Brightness is a measure of how much light is reflected by paper under specified conditions and is usually reported as a percentage of how much light is reflected, so a higher number represents a brighter or whiter paper.
Whereas the results are the same, the processes and fundamental chemistry involved in bleaching chemical pulps (like kraft or sulfite) are very different from those involved in bleaching mechanical pulps (like stoneground, thermomechanical or chemithermomechanical). Chemical pulps contain very little lignin while mechanical pulps contain most of the lignin that was present in the wood used to make the pulp. Lignin is the main source of color in pulp due to the presence of a variety of chromophores naturally present in the wood or created in the pulp mill.
Mechanical pulp retains most of the lignin present in the wood used to make the pulp and thus contain almost as much lignin as they do cellulose and hemicellulose. It would be impractical to remove this much lignin by bleaching, and undesirable since one of the big advantages of mechanical pulp is the high yield of pulp based on wood used. Therefore, the objective of bleaching mechanical pulp (also referred to as brightening) is to remove only the chromophores (color-causing groups). This is possible because the structures responsible for color are also more susceptible to oxidation or reduction.
Alkaline hydrogen peroxide is the most commonly used bleaching agent for mechanical pulp. The amount of base such as sodium hydroxide is less than that used in bleaching chemical pulps and the temperatures are lower. These conditions allow alkaline peroxide to selectively oxidize non-aromatic conjugated groups responsible for absorbing visible light. The decomposition of hydrogen peroxide is catalyzed by transition metals, and iron, manganese and copper are of particular importance in pulp bleaching. The use of chelating agents like EDTA to remove some of these metal ions from the pulp prior to adding peroxide allows the peroxide to be used more efficiently. Magnesium salts and sodium silicate are also added to improve bleaching with alkaline peroxide.
Sodium dithionite (Na2S2O4), also known as sodium hydrosulfite, is the other main reagent used to brighten mechanical pulps. In contrast to hydrogen peroxide, which oxidizes the chromophores, dithionite reduces these color-causing groups. Dithionite reacts with oxygen, so efficient use of dithionite requires that oxygen exposure be minimized during its use.
The brightness gains achieved in bleaching mechanical pulps are temporary since almost all of the lignin present in the wood is still present in the pulp. Exposure to air and light can produce new chromophores from this residual lignin. This is why newspaper yellows as it ages.
Chemical pulps, such as those from the kraft process or sulfite pulping, contain much less lignin than mechanical pulps, (<5% compared to approximately 40%). The goal in bleaching chemical pulps is to remove essentially all of the residual lignin, hence the process is often referred to as delignification. Sodium hypochlorite (household bleach) was initially used to bleach chemical pulps, but was largely replaced in the 1930s by chlorine. Concerns about the release of organochlorine compounds into the environment prompted the development of Elemental Chlorine Free (ECF) and Totally Chlorine Free (TCF) bleaching processes.
A variety of other bleaching agents have been used on chemical pulps. They include peroxyacetic acid, peroxyformic acid, potassium peroxymonosulfate (Oxone) and dimethyldioxirane which is generated in situ from acetone and potassium peroxymonosulfate, and peroxymonophosphoric acid.
Enzymes have also been proposed for use in pulp bleaching, mainly to increase the efficiency of other bleaching chemicals. It has been suggested that the process of delignification of the pulp (removing lignin to achieve white color) can be supported by oxidoreductase enzymes such as laccases.
Laccase-catalyzed oxidative delignification of kraft pulp possibly offers some potential as a replacement or booster for conventional chemical bleaching (Bourbonnais et al., 1997, “Reactivities of Various Mediators and Laccases with Kraft Pulp and Lignin Model Compounds”. Appl. Environmental Microbiol. 63: 4627-4632).
However, at present, the use of commercially available laccases is hampered because they work only in acidic conditions and ambient temperatures, whereas chemical pulping such as Kraft pulping, as well as bleaching is carried out in alkaline conditions and elevated temperatures. Most if not all commercially available laccases are of fungal origin and oxidise substrates only in acidic or neutral conditions. This requires the acidification of pulp after the bleaching process in order for the laccases to work.
There is a need for improved reagents depolymerizing lignin or bleaching reagents, in particular enzymes that work and are stable in alkaline conditions.